A nationwide U.S. study reveals that estrogen replacement therapy is risky for postmenopausal women. It increases their chance of stroke, but does not lower their risk for heart disease, as once thought.

Estrogen therapy has been available to women after menopause for 60 years, to prevent certain diseases of aging. It substitutes for declining natural production of the female hormone. Its proven benefits include maintaining strong bones and preventing vaginal degeneration. Some studies have also suggested that estrogen therapy reduces heart disease riskby one-third to one-half.

Now, a new study of 10,000 American women aged 50-79 confirms the bone benefit, but shows that estrogen treatment offers no heart disease benefit. It does, however, significantly increase the risk of stroke. University of Florida cardiologist Marian Limacher led the study, which was halted in February, one year early because of the stroke finding.

"These results suggest that for every 10,000 women followed for one year there would be 12 additional strokes in women who were taking estrogen alone, than in women who were not," she said. "That translates to about a 40 percent increased risk of having a stroke."

The stroke finding, reported in the Journal of the American Medical Association, is similar to another study conducted by the same researchers that looked at the effects of estrogen combined with progestin. That investigation was stopped early two years ago because of this and because, unlike estrogen alone, the combination also led to a higher risk for heart disease and breast cancer.

However, like estrogen alone, estrogen plus progestin decreased the risk of the brittle bone disease osteoporosis. It showed the added benefit of reduced colon cancer.

For these reasons, U.S. government health officials are not recommending a ban on hormone treatments for women, but they are urging caution. Dr. Barbara Alving is acting director of the U.S. government agency that oversaw the estrogen study, the National Heart, Lung and Blood Institute.

"They should be used at the lowest doses for the shortest possible time," she said. "If you have been on hormones for years and years and years because you just felt good, and didn't bother to go off them, and thought you were getting better cardiovascular protection, you really need to think about [whether it is] time to go off the hormones, and discuss this with your physician and gradually taper off."

Dr. Alving says two years should be the limit for estrogen therapy.

The National Heart, Lung and Blood institute official who helped guide the study, Dr. Jacques Rossouw, points out that if women decide to get hormone treatments to combat osteoporosis, there are ways they can protect themselves from increased stroke risk. He said, "In consultation with a physician and taking care of risk factors that you can take care of, such as blood pressure, there would be a place for estrogen for osteoporosis prevention."

Dr. Rossouw says studies are beginning to show that postmenopausal women's bones can be kept strong with lower doses of estrogen than doctors now recommend. But he adds that the finding about increased stroke risk should stimulate research into new osteoporosis drugs.